You can't. Muriatic acid is the only way to treat that problem, as far as I know, and the rain run-off will kill the plants. If they are not in mortar you can try turning them upside down.

Here is a link that might be useful: Propagating Perennials

I get the same thing on mine and I use a scrub brush and dishsoap. It takes some muscle but it gets alot of it off. Then I put bleach on a sponge (use gloves) and dab it onto the rocks. After about 10 miutes I wipe the bleach off, using clean rags over and over alternating damp rags with dry, until the bleach is gone. You don't want to leave it on because the rain will wash it into the other things. This keeps the algae off of the stepping stones for a month or so. Just be sure you get all of the bleach off. They also make a product to remove this algae, it's in an organic gardening catalog, but I can't think of the name of the catalog right now. Something I am going to try this year is the algae killer that I use in my water garden. It doesn't harm other vegetation, but I am not sure if it will kill this type either.
Kathy